1. Field of the Invention
The present invention relates to a servo valve for a power assisted steering gear in which an input shaft and an output shaft are connected to each other through a torsion bar.
2. Description of Related Art
In a servo valve for this type of power assisted steering gear, an input shaft is connected to a steering wheel operated by an operator. An output shaft is connected to a wheel of a vehicle through a rack-and-pinion mechanism, for example. The input shaft and output shaft are connected to each other through a torsion bar.
When steering, a flow path of pressure oil is switched depending on a relative displacement between the input shaft and the output shaft which is produced when the torsion bar is twisted. The switching of the flow path causes the pressure oil to be fed and discharged to and from a power cylinder, to obtain an auxiliary steering force.
On the other hand, when in a non-steering condition (that is, when traveling straight where the servo valve is in its neutral state), the torsion bar is hardly twisted by an operating force from the input shaft. However, a slight force from the wheel, for example, a force due to vibration or the like generated by irregularities in a road surface, or unbalance or the like between tires may, in some cases, be transmitted from the output shaft, so that the torsion bar is slightly twisted. A twist thus produced may make the travel direction unstable because it slightly operates the servo valve.
There has been conventionally provided a servo valve for a power assisted steering gear so adapted that in order to increase steering rigidity in its neutral state, there is provided a plunger the penetrates an input shaft in the radial direction. The plunger is pressed against an intermediate part in the axial direction of the torsion bar by oil pressure from the exterior, depending on the traveling state (see Japanese Patent Publication No. 41851/1978 and Japanese Patent Publication No. 41787/1986, for example).
However, a hydraulic mechanism for loading hydraulic oil pressure into the plunger, a mechanism for controlling operations performed by the hydraulic mechanism depending on the traveling state, and the like are required. Therefore, the construction is considerably complicated, and a large space is required, so that the servo valve is increased in size.